delete_user.sh

#!/bin/bash

SERVER="matrix.mycompany.com"

echo -e "\e[97mEnter user you'd like to deactivate\e[0m";
read user

if  [ "$user" == "" ]; then
        echo "User required.";
        exit;
fi

account="@$user:$SERVER";

su postgres -c "psql synapse -c 'DELETE FROM users WHERE name='\''$account'\'';'"
su postgres -c "psql synapse -c 'DELETE FROM user_directory WHERE user_id='\''$account'\'';'"
su postgres -c "psql synapse -c 'DELETE FROM account_data WHERE user_id='\''$account'\'';'"
su postgres -c "psql synapse -c 'DELETE FROM profiles WHERE user_id='\''$user'\'';'"
su postgres -c "psql synapse -c 'DELETE FROM user_external_ids WHERE user_id='\''$account'\'';'"

It is user_id, which afaik is always @USER:SYNAPSE

Thanks again, @disconn3ct . I've changed the script again accordingly.